Applicants claim priority under 35 U.S.C. xc2xa7119 of German Application No. 101 32 723.4 filed Jul. 5, 2001.
The invention relates to a satellite configuration for interferometric and/or tomographic remote sensing by means of synthetic aperture radar (SAR).
In a configuration involving a cluster of radar satellites, maintaining a baseline important for interferometry as stable as possible over a complete orbit is a problem. Baseline is termed the separation between two receiver satellites, a distinction being made between across-track baseline and along-track baseline. The first one is perpendicular to the velocity vector, serves to survey the ground level elevation and contributes by its component standing normal to the line connecting antenna and target point to sensing the ground level elevation. The along-track baseline designates the separation of two receiver satellites in the direction of the velocity vector (R. Schreiber et al., xe2x80x9cOverview of interferometric data acquisition and processing modes of the experimental airborne SAR system of DLRxe2x80x9d, Proc. IGARSS""99, Hamburg, Germany, June 1999, pp. 35-38).
The following is an introduction to SAR systems, subsequently extended to cover interferometric SAR systems. Synthetic aperture radar (SAR) is a remote sensing instrument finding ever-increasing application in terrestrial and extraterrestrial mapping, surveillance and inspection. One such system has a platform moving at constant speed, an antenna looking downwards to the imaged scene and a coherent radar system which transmits periodic electromagnetic pulses. The direction of movement of the platform is termed the azimuth direction, the direction slanting downwards to the scene is termed the range direction. Shown in FIG. 1 is an interferometric SAR system including two satellites S1 and S2 with the baseline B. "THgr" designates the viewing angle. A conventional SAR system consists of one platform only, e.g. a satellite. In FIG. 1 a SAR satellite S1 is depicted flying over a swath to be mapped. In this arrangement, a high-resolution map in the azimuth and range direction of the backscatter coefficient of the swath is generated by signal processing the raw data sensed on fly-over.
The SAR system consisting of a platform (satellite, e.g. S1) is configured by a second satellite (S2) into an interferometric system. The gist of interferometry is by measuring the phase difference of two SAR images obtained from differing perspectives to obtain additional information with which, for instance, an indication as to the relative difference in elevation of all targets in the swath can be derived. This phase difference is a result of the slight differences in range between target and the two antennas.
For the accuracy in elevation sensing it is the so-called baseline that is deciding. In FIG. 1 the baseline is illustrated as the line connecting S1 and S2. FIG. 1 shows precisely the case in which the baseline is perpendicular to the velocity vector of the satellite S1 and normal to the line connecting the satellite S1 and a target on the ground. This baseline is termed normal baseline, it being decisive for elevation sensing. When two satellites orbit at precisely the same altitude, in the same azimuth position and on parallel orbits, it is only the resulting normal baseline that contributes towards elevation sensing. The same applies to the case of two satellites orbiting precisely one above the other in thus forming a vertical baseline. Here too, it is only the resulting normal baseline that contributes towards elevation sensing. The end product of sensing the elevation is a so-called digital elevation model (DEM).
Referring now to FIG. 2 there is illustrated an interferometric system for along-track interferometry. In this arrangement the two satellites S1 and S2 are on the same orbit but slightly shifted in the flight direction. If the satellites are on differing orbits, along-track interferometry is likewise possible, it being, however, only the shift of the two satellites in the flight direction that is decisive for each constellation. Along-track interferometry is used for sensing the velocity and detecting moving targets. A typical example application of along-track interferometry is surface flow observation.
There are basically two possibilities for configuring an interferometric cluster of at least two satellites, namely multi-pass interferometry and single-pass interferometry. In multi-pass interferometry the site under observation is flown over with a SAR sensor temporally delayed with a slightly differing flight path depending on the requirement for along-track or across-track interferometry. Multi-pass interferometry is implemented successfully with the ERS-1 satellite, for example.
For single-pass interferometry at least two SAR sensors are needed and the site under observation is mapped by both sensors simultaneously. The advantage of single-pass interferometry is that the site under observation does not change between the individual SAR maps, ensuring high coherence between the two sets of interferometric data.
The disadvantage of single-pass interferometry is that several SAR sensors are needed which, as a rule, adds to the costs. Instead of two satellites a rigid design having a sufficiently long baseline between the SAR antennas may be used. This achievement was made use of in the SRTM mission.
A further achievement of the interferometric principle is the interferometric cartwheel (WO 99/58997 by D. Massonnet xe2x80x9cRoue interferomxc3xa9triquexe2x80x9d) in which a cluster of satellites describes a revolution about a virtual cartwheel center by using slightly elliptical orbits with different arguments of perigee. The configuration provided consists of, for example, three receiver satellites forming together a cartwheel and SAR transmitter which, more particularly, may be an already existing SAR sensor.
Referring now to FIG. 3 there is illustrated one such cartwheel center, where "THgr"sq designates the bistatic squint angle. During an orbit the individual cartwheel satellites describe a complete ellipse about the cartwheel center, both across-track (vertical) and along-track baselines occuring between the individual cartwheel satellites. Depending on the particular application the satellites most favorable for the application can be selected from the SAR receiver satellites.
For across-track interferometry, for example, the satellites always selected are those having the largest vertical (across-track) baseline. However, all along-track baselines may also be combined to enhance the performance of the along-track interferometry, the same optimization applying likewise to across-track interferometry for terrain topographie. (For along-track interferometry, for example, the satellites always selected are those having the optimum along-track baseline e.g. for water current mapping.)
The disadvantage of the cartwheel is that it cannot be optimized at the same time for along-track interferometry and across-track interferometry. The advantage is that maximum baselines (along or across, depending on the cartwheel expression) selected from a set of receiver satellites are highly stable over the complete orbit, i.e. vary little in length. A further disadvantage is that for safety reasons a large separation needs to be maintained between the cartwheel and the transmitter satellites. This separation results in a large bistatic squint and thus in a high Doppler centroid which makes signal processing very difficult and complicated.
In addition to along/across-track interferometry the cartwheel concept also offers the possibility of a super-resolution by making use of angular differences resulting from the local offset of the receiver satellites in along-track and across-track at which the target area is observed. These angular differences result in a shift of the corresponding spectra in the azimuth and range direction. Joining the two spectra in azimuth and range produces an enhanced signal bandwidth and thus an improved resolution in azimuth and range. For a cartwheel with three satellites the geometric resolution can be enhanced up to a factor of 2 as compared to that of only one receiver satellite.
The disadvantages of two-pass interferometry lie in the temporal decorrelation of the two SAR images needed for interferometry. Although large baselines and thus good elevation sensitivity is provided in principle, the elevation accuracy suffers from temporal decorrelation.
Temporal decorrelation is no problem in single-pass interferometry because of the simultaneous fly-over of two SAR receivers. It is, however, the restricted length of the baseline that is of disadvantage which results in a restricted elevation sensitivity. Single-pass interferometry was achieved on the STRM mission by means of a second receiver antenna mounted on a 60 m long mast. However, a structure of this length already results in vibration problems which, although corrected in part, cause serious structural and cost-effective problems in even longer structures.
The cartwheel concept combines the possibility of a baseline of almost any size in across-track for a high elevation sensitivity whilst avoiding the temporal decorrelation as typical for single-pass interferometry. Although the cartwheel concept already makes for a major advancement in satellite platform SAR interferometry, it is the cartwheel configuration of the SAR satellites that results in the following new difficulties:
Orbit maintenance is difficult since the altitude and velocity of each satellite as part of the cartwheel varies all the time. Configuring the cartwheel structure also takes long and is risky. When a separate satellite is used as the transmitter, a large safety separation needs to be maintained away from the transmitter so as not to endanger it. When the transmitter satellite is integrated in the cartwheel it is possible to position it in the center of the cartwheel ellipse formed by the receiver satellites. There is generally a high risk of collision between the individual satellites making up the cartwheel and the transmitter satellite due to the problematic orbit constellation.
It is this large separation from the transmitter satellite (in the parasitic configuration of a separate transmitter satellite) that leads to a large squint angle formed by the lines connecting the transmitter to the target and the receivers to the target. This large squint angle produces Doppler centroids in the individual data sets which amount to several multiples of the PRF vals used, thus resulting in major difficulties in interferometric signal processing.
The along-track separation of the receiver satellites from each other results in a difference in the squint angle, as a result of which the azimuth bandwidth available for interferometric applications is substantially restricted (for example in the case of transmitter satellites having a high range bandwidth since this in principle makes a large across-track baseline possible).
Across-track interferometry and along-track interferometry cannot be simultaneously optimized. Across-track interferometry can be optimized by correspondingly selecting the diameter of the cartwheel ellipse. This, however, also defines the along-track baseline important for along-track interferometry. A favorable vertical baseline, for example, for the C band is in the range of one to several kilometers, resulting in maximum along-track baselines of likewise several kilometers. Due to the motion of the receiver satellites on the cartwheel ellipse both the across-track and the along-track baseline vary.
To avoid calibration points it is necessary to calibrate the complete system over the sea as was done, for example, on the SRTM mission. Therefore, the along-track separation of the receiver satellites suitable for calibration should be below the so-called correlation time for water surfaces.
This correlation time is a time duration derived from the roughness of the sea surface within which the two SAR images need to be mapped to ensure a useful coherence. For the C band this time duration is of the order of 57 ms, corresponding to an along-track separation of two satellites of roughly 400 m at an altitude of approximately 800 kilometer. Due to the variation of the along-track baseline with maxima of up to several kilometers for an optimized across-track baseline of the cartwheel, calibration over the sea is very difficult to achieve.
The invention is based on the objective of maintaining baselines over an orbit in a configuration involving a cluster of satellites as stable as possible. Furthermore, the satellite configuration is intended to be configured so that along-track interferometry and across-track interferometry can be implemented and optimized simultaneously.
In accordance with the invention this objective is achieved by a satellite configuration for interferometric and/or tomographic remote sensing by means of synthetic aperture radar (SAR) wherein one to N receiver satellites and/or transmitter satellites and/or transceiver satellites with (or partly with) a horizontal across-track shift the same or differing in amplitude are provided, the satellites orbiting at the same altitude and at the same velocity, and a horizontal along-track separation, constant irrespective of the orbital position, being adjustable between the individual receiver satellites.
For achieving the objective as well as solving the problems associated with prior art there is provided in accordance with the invention a constellation of a cluster of SAR receiver satellites, all orbiting, for example, on circular orbits the same in altitude and velocity. The required across-track baseline is achieved by the inclinations and/or ascending nodes of the individual receiver satellite orbits differing slightly.
An ascending node is the intersection of the equatorial plane and the orbit of the satellite on its way from the southern hemisphere to the northern hemisphere. Different ascending nodes of the receiver satellites result in a horizontal across-track shift which varies twice during an orbit between a maximum value and zero, the maximum of the horizontal across-track shift occurring over the equator while over the poles the orbits intersect. However, the maxima of the horizontal across-track shift may also be placed over the poles, the ascending nodes then being identical and the inclinations slightly differing. The maximum may also be positioned otherwise.
At the same time and independent of the across-track baseline an along-track baseline may be set by a minor shift in time of the receiver satellites in the direction of flight.
The constellation in accordance with the invention is also termed in the following cross-track pendulum and offers the following advantages:
Any temporal decorrelation is excluded due to the practically simultaneous fly-over of all receiver satellites.
An across-track baseline of any desired length can be formed as regards the requirements on across-track interferometry without having to take into account any structural restrictions of a cohesive construction.
Similar to the situation in a cartwheel configuration a highly stable maximum across-track baseline is attained, the maximum varying only between 87% and 100% of the absolute maximum when using three satellites.
Unlike the cartwheel configuration, the along-track baseline can be optimized irrespective of the across-track baseline, for example in oceanographic applications.
Orbit maintenance becomes very simple due to all receiver and transmitter satellites orbiting at the same altitude and velocity, thus greatly reducing the risk of a collision, unlike the situation in a cartwheel configuration.
The along-track separation between a transmitter satellite separate to the cross-track pendulum or one belonging thereto can now be configured substantially less than in the cartwheel concept due to the lower risk of collision. This results in the solution of the problem with high squint angles as occurring in the cartwheel concept in thus substantially facilitating interferometric signal processing of the satellite data which can now be done with standard methods of SAR interferometry.
Due to the independent optimization of the baselines for across-track interferometry and along-track interferometry, several missions can now be implemented in parallel. For example, with across-track interferometry a global terrain model can be generated whilst simultaneously implementing along-track interferometry for sensing the velocity of ocean currents or super-resolution.
Although maintaining the orbits of the satellites slightly differing in inclination necessitates additional fuel this is not a technological limitation. For example, the additional annual fuel requirement of a pendulum configuration with ENVISAT as illuminator amounts to only approximately 1% of the receiver satellite weight. By comparison, reference is made to the cartwheel mission in which fuel consumption is approximately 4.5% of the receiver satellite weight (approximately 2 year mission duration) for orbit control of the complete mission.